The world's second-wealthiest man explains to CNBC why workers in Europe and
other parts of the developed world are at the "best" part of their career after they
reach their 60s.

Billionaire Carlos Slim, age 73, says workers -- those in developed economies, anyway -- are at their prime in their 60s.

"When you have an industrial economy like in the past where people ... do a lot of physical work and people live less years, it's OK to retire at 65," Slim told CNBC. "When you have a society of knowledge and experience and information, at this age is where you are at your best. It's [foolish] to retire at this age. And you don't have the physical work, and you have the intellectual work and you are in your best in your 60s."

The comment came up in a discussion about what to do to fix the European economy. Many economists and politicians have said retirement ages around the world need to rise because people are living longer and the benefits promised to retirees are unsustainable. However, Slim said, even beyond that, workers are better in their 60s than when they're younger.

Another way he would fix Europe: split the work week.

"Today they are working 35 hours. Now when you talk about 9 to 5, it's five days, eight hours. But with a coffee break and lunch etc., they work seven hours. They are working at most 35 hours. Maybe it's better to work only three days for 11 hours and have four days and this way others will work," he said.

Slim's theories about the value of older workers bring may have something to them, if worker pay is any indication. A recent analysis of U.S. government data form the Brookings Institution found that people who still work near or past their traditional retirement age earn significantly more per hour than their younger colleagues.

That report, which was funded by the Social Security Administration, found that 60- to 74-year-old men earned $25.12 an hour, on average, in 2011. That's over 20 percent more than the average hourly wage of working men 25 to 59. Older women also out-earned their younger counterparts.